In recent years there have been many advances in the production of polyolefin homopolymers and copolymers due to the introduction of metallocene catalysts.
Metallocene catalysts offer the advantage of generally a higher activity than traditional Ziegler catalysts and are usually described as catalysts which are single site in nature.
There have been developed several different families of metallocene complexes. In earlier years catalysts based on bis (cyclopentadienyl) metal complexes were developed, examples of which may be found in EP 129 368 or EP 206 794. More recently complexes having a single or mono cyclopentadienyl ring have been developed. Certain of such monocyclopentadienyl complexes have been referred to as constrained geometry complexes and examples of these complexes may be found in EP 416 815 or EP 420 436. In both of these complexes the metal atom e.g. zirconium is the highest oxidation state.
Other complexes however have been developed in which the metal atom may be in a reduced oxidation state. Examples of both the bis (cyclopentadienyl) and mono(cyclopentadienyl) complexes have been described in WO 96/04290 and WO 95/00526 respectively.
The above metallocene complexes are utilized for polymerization in the presence of a cocatalyst or activator. Typically activators are aluminoxanes, in particular methyl aluminoxane or compounds based on boron compounds. Examples of the latter are borates such as trialkyl-substituted ammonium tetraphenyl or tetrafluorophenyl borates. Catalyst systems incorporating such borate activators are described in EP 516 479, EP 418 044 and EP 551 277.
The above metallocene complexes may be used for the polymerization of olefins in solution, slurry or gas phase. When used in the gas phase the metallocene complex and/or the activator are suitably supported. Typical supports include inorganic oxides (e.g. silica or alumina) or polymeric supports may alternatively be used.
Examples of the preparation of supported metallocene catalysts for the polymerization of olefins may be found in WO 94/26793, WO 95/07939, WO 96/00245, WO 96/04318, WO 97/02297 and EP 642 536.
WO 98/27119 describes supported catalyst components comprising ionic compounds comprising a cation and an anion in which the anion contains at least one substituent comprising a moiety having an active hydrogen. In this disclosure supported metallocene catalysts are exemplified in which the catalyst is prepared by treating the aforementioned ionic compound with a trialkylaluminum compound followed by subsequent treatment with the support and the metallocene.
WO 98/27119 also describes a method for activating a substantially inactive catalyst precursor comprising (a) an ionic compound comprising a cation and an anion containing at least one substituent comprising a moiety having an active hydrogen; (b) a transition metal compound; and optionally (c) a support by treatment with an organometallic compound thereby forming an active catalyst.
Various methods have been utilized to prepare supported catalysts of this type.
For example, WO 98/27119 describes several methods of preparing the supported catalysts disclosed therein in which the support is impregnated with the ionic compound. The volume of the ionic compound may correspond from 20 volume percent to greater than 200 volume percent of the total pore volume of the support. In a preferred preparative route the volume of the solution of the ionic compound does not exceed substantially, and is preferably equal to, the total pore volume of the support.
Such methods of preparation may be referred to as incipient precipitation or incipient wetness techniques.
More recently WO 02/06357 describes an improved incipient wetness technique for the preparation of a supported metallocene catalyst system in which the support is impregnated with an ionic compound and the metallocene complex followed by treatment with an organometallic compound.
The preferred metal with respect to the organometallic compound is aluminum and the preferred metal for the ionic activator is boron whereby the molar ratio of Al/B is in the range of 0.1 to 2.0, and is preferably in the range of 0.1 to 0.8, and most preferably in the range of 0.3 to 0.6.
The “ionic activators” described above are highly desirable for slurry and gas phase olefin polymerizations especially for the preparation of catalysts which are not as prone to cause reactor fouling as similar catalysts prepared with aluminoxanes. One challenge that remains is to increase the activity/productivity of catalysts that incorporate these activators. We have discovered that certain catalyst molecules having cyclopentadienyl and phosphinimine ligands exhibit excellent activity when the cyclopentadienyl ligand is substituted. The level of substitution should include at least 2 carbon atoms. A preferred maximum level of substitution is 25 carbon atoms (on the 5 carbon atom ring of the cyclopentadienyl ligand) due to steric considerations.